Geologists typically make geologic maps for other geologists. The geologic vocabulary of these maps, like any jargon, simplifies communication among geologists, but effectively conceals the geologic information with its rich properties from non-geologists. An expert system in a GIS provides an automated process of translating the geologic properties associated with lithology and geologic time into a wide spectrum of derivative maps in the language of non-geologists. The specific expert system we use requires geologic maps stored in the ArcGeology data model, but the expert approach could be applied to any geologic-map data model.